The following sections of this BookRags Literature Study Guide is offprint from Gale's For Students Series: Presenting Analysis, Context, and Criticism on Commonly Studied Works: Introduction, Author Biography, Plot Summary, Characters, Themes, Style, Historical Context, Critical Overview, Criticism and Critical Essays, Media Adaptations, Topics for Further Study, Compare & Contrast, What Do I Read Next?, For Further Study, and Sources.
(c)1998-2002; (c)2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc. Gale and Design and Thomson Learning are trademarks used herein under license.
The following sections, if they exist, are offprint from Beacham's Encyclopedia of Popular Fiction: "Social Concerns", "Thematic Overview", "Techniques", "Literary Precedents", "Key Questions", "Related Titles", "Adaptations", "Related Web Sites". (c)1994-2005, by Walton Beacham.
The following sections, if they exist, are offprint from Beacham's Guide to Literature for Young Adults: "About the Author", "Overview", "Setting", "Literary Qualities", "Social Sensitivity", "Topics for Discussion", "Ideas for Reports and Papers". (c)1994-2005, by Walton Beacham.
All other sections in this Literature Study Guide are owned and copyrighted by BookRags, Inc.
Certain cells within the nervous system are called "excitable cells." Surrounding each of these cells is a highly complex, semipermeable membrane, an insulating barrier which separates fluids inside the cell from fluids outside. In the quiet state, when the cell is resting or unstimulated, the inside of a neuron's membrane is negatively charged--about -70 millivolts (mV)--and the outside is positively charged. This difference in voltage or charge is stored in the membrane and called the membrane potential because it provides a source of potential energy which can be used to generate and propagate electrical signals from one cell to the next through ion currents. The membrane is selectively permeable to ions, particularly potassium (K+), which is more highly concentrated inside the cell, and sodium (Na+), more highly concentrated outside the cell. When an event excites the cell, these ions change places through voltage-gated channels--or special pores--in the membrane. The inner and outer membrane charges reverse, triggering an action potential, an all-or-none response which travels down the cell's axon to the synaptic terminals where it causes the release of neurotransmitters into the synapse. These neurotransmitters, in turn, stimulate the adjoining cell. Neurons have a threshold stimulus, which means there is a minimum level of stimulation needed to trigger the exchange of K+ and Na+ ions across the membrane. They also need a refractory period, time during which the K+ and Na+ to return to their normal intra and extracellular levels, once again giving the membrane its potential to fire off another signal.